Filter medium and method of making same

ABSTRACT

A filter medium particularly adapted for use in an oil filter for an internal combustion engine having a water-laid web of fibers, at least 70% of which are cellulose, impregnated with resin in a pattern at its wire side to a depth of 15 to 45% of its total thickness, the pattern having zones free from resin amounting to 35 to 60% of the total area of the wire side, each zone having at least one dimension no greater than 4 mm.

This application is a continuation-in-part of our pending applicationSer. No. 691,420 filed June 1, 1976, now abandoned.

This invention relates to a filter medium and pertains more specificallyto a filter medium particularly useful in a pleated filter forhydrocarbon oils, as in an oil filter for a internal combustion enginesuch as a diesel or gasoline engine, and it can also be used in airfilter or fuel filter for such engines.

It has previously been proposed in McNabb et al. U.S. Pat. No. 3,116,245to provide a filter medium by first making a water-laid web of speciallyselected cellulose fibers of high purity, then impregnating the web fromthe wire side with a resin which penetrates approximately one-half thethickness of the web. It has also been proposed in Briggs U.S. Pat. No.3,096,230 to provide a filter medium by impregnating a paper withthermosetting resin to approximately one-third of its thickness andcoating with cold pressed castor oil. It has also been proposed inKeedwell U.S. Pat. No. 3,520,416 to impregnate a fibrous filter sheet ina pattern extending entirely through the sheet with selected materialsincluding resins so that some zones of the sheet are liquid repellentand others are liquid wetting, the impregnated sheet consequently beingpermeable to gas even after saturation with liquid. Moreover, in thecase of fibrous sheet products particularly useful as electricalinsulation, it has been proposed, in Ford U.S. Pat. No. 2,399,338 toapply laminating adhesive to one face of a fibrous sheet in a pattern ofstripes, and in Grimes U.S. Pat. No. 3,203,823, Leonard et al. U.S. Pat.No. 3,071,845, and Ford U.S. Pat. No. 3,246,271, it has been proposed toapply adhesive resin to one or both sides of a fibrous sheet ofelectrical insulation in various patterns.

Moreover, fibrous sheets impregnated throughout their area to a depth ofabout 100% of their thickness have been widely used in a pleated annularconfiguration such as that shown in McNabb U.S. Pat. No. 3,116,245 asoil filters for internal combustion engines; extensive standard testspecifications have been devised for evaluating the performance of oilfilters, indentified as SAE J-806a (Rev. 1972). Among the more importantcharacteristics measured by these tests are (1) contaminant holdingcapacity, i.e., the quantity of solid contaminant particles which thefilter is capable of removing from flowing oil and holding withoutexcessive increase in pressure drop through the filter; (2) filteringefficiency, i.e., the percentage of finely-divided solid contaminantparticles removed from oil passage; (3) media migration, i.e., thequantity of contaminant material introduced into the flowing oil streamby its passage through the filter, comprising primarily small fibers orparticles released from the filter medium; (4) collapse, i.e., tendencyof the pleated filter unit to collapse or tear because of highdifferential pressure due to cold oil or accumulation of contaminant onor within the filter medium during use or both. Experience has indicatedthat alteration of a resin-impregnated fibrous web filter medium in aneffort to improve its performance with respect to one or more of theforegoing characteristics has resulted in a decrease in its performancewith respect to other remaining characteristics. Because of theapparently mutually exclusive or contradictory nature of therequirements to improve the performance of filter media with respect toall of the foregoing characteristics, little progress has been made inoverall improvement of the filter medium since widespread adoption ofresin-impregnated fibrous web filter media.

The present invention comprises a filter mediun which provides not onlygreatly improved contaminant holding capacity and efficiency as comparedto commercially available oil filter media impregnated with resinthroughout their entire surface area to the full extent of theirthickness, but at the same time meets the requirements for minimal mediamigration and resistance to collapse. The substantial improvement insome characteristics demonstrated by the filter medium of the presentinvention without concomitant substantial worsening of one or more ofthe remaining characteristics renders the filter medium unique. Thefilter medium of the invention comprises a water-laid web of fibersincluding at least 70% by weight of cellulose fibers, the web having aream weight of 35 to 180 lb., (3,000 sq.ft. ream), the web beingimpregnated with resin in a pattern at its wire side to a depth from 15to 45% of the total thickness of the web, the amount of the resin beingfrom 9 to 15% by weight of the web, preferably from 11 to 15% by weightof the web, the pattern having first zones free from the resin and otherzones impregnated with the resin, at least one surface dimension of eachof the first said zones being from 0.1 to 4 mm., the total area of thefirst said zones being from 35 to 60% of the total surface area of thewire side, and the air porosity of the impregnated web being from 1 to200 CFM per sq.ft. at a pressure drop of 0.5 in. water on a FrazierPorosity Tester.

The water-laid fibrous web of the present invention can be made on aFourdrinier machine or on a Rotoformer machine; in either case, it isthe side next to the Fourdrinier wire or next to the Rotoformer wire, asthe case may be, called the "wire side", which is resin-impregnated. Thefibers in the furnish used to make the web include at least 70% byweight of cellulose fibers. These may be alpha cellulose fibers of highpurity if desired, but low purity (having an average alpha cellulosecontent less than 90% by weight) may also be used, and a filter mediummade of such low purity fibers is particularly important because thepresent invention makes it possible to upgrade very markedly thecontaminant holding capacity of a filter medium containing such fibersas compared to a filter medium of the prior art made from comparablefibers. For example, there can be used as the cellulose fibers in thepresent invention, fibers from cotton linters, wood pulp, reclaimedpaper fibers, and other cellulose fibers from similar sources, eitherbleached or unbleached. Various synthetic fibers may also be present inamounts up to 30% by weight of the total fibers; among suitablesynthetic fibers are those of glass, rayon, nylon, polyester or acryliccomposition and the like. Such synthetic fibers, when present, arepreferably of dimensions from 0.5 to 20 denier cut 3.2 to 12.7 mm. inlength.

In the accompanying drawing,

FIG. 1 is a plan view of one embodiment of the invention as viewed fromthe wire side of the web on an enlarged scale, machine direction of theweb being shown by the arrow, and,

FIG. 2 is a view in cross-section taken along line 2--2 of FIG. 1.

The water-laid web of fibers should have a ream weight (3,000 sq.ft.) of35-180 lb. There may be present, if desired, any of the usual wetstrength resins in the usual amounts; for example, an acid colloidmelamine resin or an anionic or cationic urea resin in the usual amountfrom 0.5 to 2.5% based on the total fiber content when added in thebeater or 5 to 7% based on the cured weight of the web when added in asaturator. This relatively small amount of wet strength resin, when itis present, is uniformly distributed throughout the web and helps tominimize fiber migration in the finished web.

The water-laid web of fibers, following its formation, is dried in theusual manner, then impregnated with resin in a pattern at its wire sideto a depth from 15 to 45% of the total thickness of the web. The resinemployed may be any desired thermoplastic or thermosetting resin capableof acting as a binder or reinforcing agent, including such resins asphenol-formaldehyde, urea-formaldehyde, melamine-formaldehyde, polyvinylacetate, polyvinyl chloride, polyacrylates, and combinations of any oneor more of the foregoing. Of these, liquid thermosetting resins,particularly phenol-formaldehyde resins, are often preferred. The resincan be applied to the web with a liquid vehicle in the form of asolution of the resin in an organic solvent, but water is the preferredvehicle, the resin being applied in the form of an aqueous mixture. Ineither case, in order to attain the desired depth of penetration fromthe wire side of the web, it is preferred to adjust the viscosity of theresin solution or mixture to the range from 8,000 to 50,000 centipoises.This high viscosity makes it possible to use a high total solids resinsolution or mixture, thus minimizing the amount of liquid vehicle to beevaporated and also minimizing the cost of manufacture.

The amount of resin (dry solids content) with which the web ispattern-impregnated should be from 9 to 15% by weight of the web towhich it is applied (the weight of the web including any wet strengthresin present in the web in addition to the fibers).

The pattern in which the resin is applied should be one which provideszones of the web, viewed from the wire side, which are free from resin,and other zones which are impregnated with resin; the total area of thezones free from resin should be from 35 to 60% of the total surface areaof the wire side of the web. The pattern may be such that either theimpregnated or non-impregnated zones are in the form of stripes or ofdiscontinuous discrete areas, one of which is surrounded by the other,as in a grid, for example. In the embodiment shown in FIG. 1 of thedrawing, the areas 10 are impregnated with resin, while the spaces 12between them are free from resin. Either of the zones may be of anydesired geometrical shape or configuration such as circles, ellipses,annuli, rectangles, triangles, regular or irregular polygons or evenmore complex designs including those which have aesthetic value or whichconvey information such as monograms, words, or pictures, provided onlythat each unimpregnated surface zone has at least one surface dimensionwhich is from 0.1 to 4 mm. For this purpose, each such zone isconsidered to have only two surface dimensions, measured at right anglesto each other. Stated another way, the size and shape of each surfacezone which is free from resin must be such that the distance from anypoint on the surface of each zone to the nearest edge of the zone mustbe from 0.05 to 2 mm. In the case of a pattern in which one dimension ofthe resin free zone is greater than 4 mm., the other being 4 mm. orless, it is preferred that the pattern should be arranged so that thelonger dimension should extend transversely of the machine direction ofthe web, while the shorter dimension extends parallel to, or more nearlyparallel to, the machine direction of the web. For example, in the caseof a pattern of straight parallel stripes, preferably the stripes extendat an angle from 35° to 90° from the machine direction of the web.

After the pattern impregnation of the web with resin, the impregnatedweb may be heated to dry it and to effect such partial advancement ofthe resin as may be desired, in the case of thermosetting resins. Inthis form it can be stored or shipped. In the case where the web is tobe subsequently folded or otherwise shaped to fit into a housing, forexample, in the case of a pleated filter cartridge for lubricating oil,as shown in McNabb et al. U.S. Pat. No. 3,116,245, the filter medium maybe subsequently heated to facilitate pleating, after which it may begiven a final cure in the case of thermosetting resins. In the case of aweb impregnated with a phenol-formaldehyde resin, the resin may beadvanced to the B-stage with the web in flat condition, after which itit may be cut to size, pleated and shaped, then the resin may be curedto the desired C-stage.

A filter medium made in accordance with the present invention displaysan air porosity or permeability from 1 to 200 CFM per sq.ft. at apressure drop of 0.5 in. water on a Frazier Porosity Tester understandard conditions as described by Schiefer et al., J. Research Nat.Bur. Standards, Vol. 28, 637-642 (1942), and is useful either as an airfilter or as a liquid filter.

In using the filter medium of the present invention, the impregnated orwire side of the web should be the downstream side.

EXAMPLE 1

A series of fibrous sheets or webs was prepared on a Rotoformer machinefrom an aqueous furnish containing 100% cotton linter fibers (fiberlength 2-4 mm.) having different ream weights as set forth in thefollowing Table I. Wet strength resin (0.7% based on the dry weight ofthe fiber) was introduced into the slurry of fibers. After formation anddrying of the web in the usual manner, the wire side of severaldifferent samples was pattern-impregnated on the wire side with anaqueous mixture of 45% by weight of phenol-formaldehyde resin (Aerotap557) and 55% polyvinyl acetate (Polyco 2113) thickened with polyacrylate(Rhoplex ASE 60) to provide a viscosity of 20,000 cps. The resin mixturewas applied to the wire side of each sample in a grid pattern as shownin FIG. 1 on a gravure roll, dried, and heated at 130° C. for 1 to 3minutes to advance the phenol-formaldehyde resin to the B-stage.Sectioning of the finished product showed that the resin penetratedapproximately 30% of the total thickness of the web from the wire side.The grid pattern consisted of an impregnated zone in the form of rows ofsquares, each square being 2 mm. on a side and spaced 0.7 mm. from eachadjacent square, each row extending at 45° to the machine direction ofthe web as shown in FIG. 1. The zone which was free from resin (exceptfor the wet strength resin which was uniformly distributed throughoutthe web) formed a pattern of two sets of parallel stripes intersectingat right angles to each other. Other samples were prepared from the samefiber furnish at various ream weights with no resin patternimpregnation, and still other samples were impregnated at the wire sidewith the same resin using complete coverage of the wire side andpenetration to about 50% of the web thickness.

The samples were pleated, the resin cured to the C-stage by heating for15 minutes at 150° C. and tested by subjecting them to a flow oflubricating oil containing AC Test Dust as a contaminant in accordancewith the standard oil filter test procedure, SAE J-806a (REV. 1972) todetermine capacity and efficiency. The results were as follows:

                                      TABLE I                                     __________________________________________________________________________            Ream       Percent                                                            wt. of                                                                              Pleat                                                                              Resin based                                                                           Test Capacity,                                                                           Efficiency                              Impregnation                                                                          web, lb.                                                                            Count                                                                              on cured web                                                                          Hours                                                                              grams Percent                                 __________________________________________________________________________    No resin                                                                              76    58   None    6.5  6.70  37.7                                    "       76    58   None    8.75 8.34  54.0                                    Pattern 83    58   11.6    13.5 15.28 68.2                                    "       83    58   11.6    13.25                                                                              14.94 62.5                                    Pattern 85    68   10.8    17.25                                                                              20.81 92.1                                    "       "     "    "       15.75                                                                              19.51 94.9                                    Pattern 108   53   12.6    16.0 19.37 87.8                                    "       "     "    "       15.25                                                                              17.40 87.6                                    Complete                                                                      coverage                                                                              89    68   13.5-15.5                                                                             10.5 10.68 80.6                                    "       "     "    "       7.0  8.09  74.0                                    Complete                                                                      coverage                                                                              95    53   13.5-15.5                                                                             11.0 12.59 84.6                                    "       "     "    "       9.5  9.77  80.4                                    Complete                                                                      coverage                                                                              112   51   13.5-15.5                                                                             16.0 13.66 79.3                                    "       "     "    "       15.0 11.66 84.5                                    __________________________________________________________________________

Other tests carried out by the same test procedure using syntheticcontaminant SOFTC 2A showed markedly higher capacity for thepattern-impregnated fibrous web of the present invention and somewhathigher efficiency than for similar webs impregnated throughout thecomplete area of the wire side to a depth of about 50% of the thicknessof the web.

Still other webs were prepared from a wood pulp furnish (89%alpha-cellulose or less); some samples of these webs were impregnated inaccordance with the present invention using the same pattern, resinformulation, and depth of impregnation or penetration as set forthabove, while other samples were fully saturated with resin so that theimpregnation extended throughout the entire area of the wire side andthrough the full thickness of the web. These samples were then testedfor capacity and efficiency by the same test procedure as describedabove, using AC Dust Contaminant, with the following results:

                                      TABLE II                                    __________________________________________________________________________            Ream       Percent                                                            wt. of                                                                              Pleat                                                                              Resin based                                                                           Test Capacity,                                                                           Efficiency,                             Impregnation                                                                          web, lb.                                                                            Count                                                                              on cured web                                                                          Hours                                                                              Grams percent                                 __________________________________________________________________________    Pattern 83/86 68   11.6    17.5 22.19 79.8                                    "       "     "    "       17.75                                                                              22.07 83.9                                    Complete                                                                              83    68   18.6    8.0  10.44 85.9                                    "       "     "    18.6    9.0  9.65  85.9                                    __________________________________________________________________________

Similar results were obtained in tests where a synthetic sludgecontaminant (SOFTC-2A) was used as the test contaminant.

All of the filter media tested satisfied the accepted industry standardfor fiber migration (SAEJ 806a) of less than 5 mg. per filter elementexcept the unsaturated webs.

EXAMPLE 2

A single fibrous web was prepared on a Rotoformer machine from anaqueous furnish in which all of the fiber content was wood cellulosefiber (Southern bleached pine), the furnish also containingapproximately 1.5-2.0% wet strength melamine-formaldehyde resin byweight based on the dry weight of the fibers, the web having a reamweight of 88-92 lb.

Several different samples of this web were pattern-impregnated on thewire side with an aqueous phenol-formaldehyde resin formulation havingthe following composition:

    ______________________________________                                                    Dry solids,                                                                              Total, including                                                   percent    water, percent                                         ______________________________________                                        Urea          3.0          0.8                                                Release agent and                                                             lubricant (Aura-                                                              soft 216, 100%                                                                active)       19.4         5.5                                                Phenol-                                                                       formaldehyde                                                                  resin (Aerotap                                                                8095 W50, 50%                                                                 solids)       51.8         29.2                                               Thickener                                                                     (Rhoplex ASE 60,                                                              28% solids)   14.5         14.5                                               Triethanolamine                                                                             11.4         3.2                                                Water         --           46.8                                               ______________________________________                                    

The aqueous resin formulation was applied using the same gravure roll asin Example 1, so that the resin content of the cured web was 10.3% byweight, or 11.4% by weight of the unimpregnated web. The impregnated webwas dried and heated at 130° C. for 1 to 3 minutes to advance thephenol-formaldehyde to the B-stage. Resin penetrated to approximately30% of the total thickness of the web from the wire side, the patternbeing the same as in Example 1 and shown in FIG. 1.

EXAMPLE 3

Other samples of the same web as described in Example 2 were impregnatedas described in that example except that the aqueous resin formulationcontained the following composition:

    ______________________________________                                                    Dry solids,                                                                              Total, including                                                   percent    water, percent                                         ______________________________________                                        Urea          1.7          0.8                                                Release agent and                                                             lubricant (Aura-                                                              soft 216, 100%                                                                active)       10.7         5.0                                                Phenol-                                                                       formaldehyde                                                                  resin (Aerotap                                                                8095 W50, 50%                                                                 solids)       28.7         26.5                                               Thickener                                                                     (Rhoplex ASE 60,                                                              28% solids)   8.0          13.1                                               Triethanolamine                                                                             6.3          2.9                                                Polyvinyl acetate                                                             (Polyco 2113) 44.7         37.7                                               Water         --           14.0                                               ______________________________________                                    

The resin solids content of the cured web was 12.1% by weight, or 13.8%by weight of the unimpregnated web. Penetration was approximately 30% ofthe total thickness of the web from the wire side, and the impregnatedweb was dried and heated as in Example 2 to provide a product having thesame pattern.

Impregnated web samples of each of Examples 2 and 3 were then pleated toform filter units of the configuration shown in McNabb et al. U.S. Pat.No. 3,116,245, the resin was cured to the C-stage by heating for 15minutes at 150° C., and the filter units were tested in accordance withChapter 2 of Oil Filter Test Procedure SAE J-806a (Rev. 1972) forcapacity and efficiency using AC Fine Test Dust as the contaminant inSAE 30 oil preheated to 180° F. and taking as the end point an increasein the pressure drop across the filter to 8 p.s.i. The results were asfollows:

    ______________________________________                                                         Capacity                                                            Life      (grams contaminant                                                  (Hours to retained by filter                                                                          Percent                                               End Point)                                                                              at end point) Efficiency                                     ______________________________________                                        Example 2                                                                              14          16.99         78.1                                                13 1/2      17.25         83.6                                       Example 3                                                                              14 3/4      18.28         82.3                                                14          17.39         81.2                                                13 7/12     17.86         83.6                                                13 1/4      18.64         82.7                                       ______________________________________                                    

Six samples of each of Example 2 and 3 were tested for media migrationin accordance with Chapter 4 of the same test procedure, the average forExample 2 being 1.6 mg/filter (total 9.6 mg for six samples) and forExample 3, 2.20 mg/filter (total 13.2 mg for six samples).

A collapse test was conducted in accordance with Chapter 5 of the sametest procedure using some of the same filter units previously used forthe Chapter 2 and Chapter 4 tests. The oil was at room temperature, thepressure being gradually increased, and the end point was taken as asudden decrease in oil pressure, indicating collapse of the center metaltube of the filter unit. Each unit was then inspected to determine thenature of the failure. In addition to collapse of the center tube, manyshowed a failure of the adhesive bond between the web and the end caps.However, in only one unit out of six of Example 2 was there a failure(fracture) in the impregnated web itself, and in none of the units ofExample 3 was there any such failure. The filter units of both Example 2and Example 3 were consequently considered satisfactory with respect toall of the tests.

The products of both Example 2 and Example 3 displayed satisfactory airporosity, that of the former being approximately 50 CFM and of thelatter approximately 40-44 CFM at a pressure drop of 0.5 in. water on aFrazier Porosity Tester.

In contrast, filter units made in accordance with the prior art failedin respect of one or more of the same tests.

What is claimed is:
 1. A filter medium particularly adapted for use inan oil filter for an internal combustion engine, said medium comprisinga water-laid web of fibers including at least 70% by weight of cellulosefibers, said web having a ream weight of 35 to 180 lb., said web beingimpregnated with resin in a pattern at its wire side to a depth from 15to 45% of the total thickness of the web, the amount of said resin beingfrom 9 to 15 % by weight of the web, said pattern having first zonesfree from said resin and other zones impregnated with said resin, atleast one surface dimension of each of the first said zones being from0.1 to 4 mm., the total area of the first said zones being from 35 to60% of the total surface area of said wire side, and the air porosity ofsaid impregnated web being from 1 to 200 CFM per sq.ft. at a pressuredrop of 0.5 inch water on a Frazier Porosity Tester.
 2. A filter mediumas claimed in claim 1 in which said first zones have a second dimensiongreater than 4 mm. and said second dimension extends transversely of themachine direction of the web.
 3. A filter medium as claimed in claim 1in which said first zones are in the form of continuous stripes.
 4. Afilter medium as claimed in claim 1 in which said first zones are in theform of discrete closed areas.
 5. A filter medium as claimed in claim 1in which said cellulose fibers are cotton linters.
 6. A filter medium asclaimed in claim 1 in which said cellulose fibers have an average alphacellulose content less than 90% by weight.
 7. A filter medium as claimedin claim 1 in which said resin comprises phenol-formaldehyde.
 8. Afilter medium as claimed in claim 1 in which the amount of the resin isfrom 11 to 15% by weight of the web.
 9. A filter medium particularlyadapted for use in an oil filter for an internal combustion engine, saidmedium comprising a water-laid web of fibers including at least 70% byweight of cellulose fibers, said web having a ream weight of 35 to 180lb., said web being impregnated with resin in a pattern at its wire sideto a depth from 15 to 45% of the total thickness of the web, the amountof said resin being from 9 to 15% by weight of the web, said patternhaving first zones free from said resin and other zones impregnated withsaid resin, the size and shape of each of the first said zones beingsuch that the distance from any point on the surface of each zone to thenearest edge of the zone is from 0.05 to 2 mm., the total area of thefirst said zones being from 35 to 60% of the total surface area of thewire side, and the air porosity of said impregnated web being from 1 to200 CFM per sq.ft. at a pressure drop of 0.5 in. water on a FrazierPorosity Tester.
 10. A filter medium as claimed in claim 9 in which saidcellulose fibers are cotton linters.
 11. A filter medium as claimed inclaim 9 in which said cellulose fibers have an average alpha cellulosecontent less than 90% by weight.
 12. A filter medium as claimed in claim9 in which said first zones are in the form of continuous stripes.
 13. Afilter medium as claimed in claim 9 in which said first zones are in theform of discrete closed areas.
 14. A filter medium as claimed in claim 9in which said resin comprises phenol-formaldehyde.